This study will test the hypothesis that Culiseta melanura serves as the primary enzootic vector of western equine encephalomyelitis (WEE) virus in the northeastern United States and will evaluate the influence of Cs. melanura on the winter survival of virus, seasonality of virus isolations, restriction of virus activity to enzootic foci and stability of antigenic specificity of virus strains from the Northeast. The proposal includes: 1) Evaluation of the vector competence of Cs. melanura and other mosquito species which have been implicated by field studies as possible vectors of WEE virus in the Northeast. 2) Examination of the ability of experimentally infected Cs. melanura to passage virus to eggs and F1 progeny. 3) Evaluation of the ability of experimentally infected Cs. melanura larvae in diapause to maintain WEE virus over an extended period of time and the effect of such maintenance on biological and immunological properties of the virus. 4) Determination of the extrinsic incubation period of WEE virus in Cs. melanura at various temperatures and the effect of temperature on the feeding behavior and oviposition cycle of this mosquito. 5) Evaluation of the differential susceptibility and transmission efficiency of Cs. melanura and Culex tarsalis for strains of WEE virus from different geographical regions of the United States and the effect of long term serial passage through these mosquitoes on the distinct antigenic character of such virus strains.